1. Field of the Invention
The present invention relates to a log conversion circuit adapted for an electronic device driven by a low source voltage, e.g., a portable communication device.
2. Description of the Related Art
A type of log conversion circuit log-converts an input signal, utilizing the fact that a base-emitter voltage and a collector current of a bipolar transistor have a logarithmic relation or that a forward voltage and a collector current of a diode have a logarithmic relation. The log conversion circuit is often used as a variable gain circuit, called a gain cell circuit, in combination with an antilog conversion circuit connected to its output side.
FIG. 1 shows a conventional gain cell circuit using a conventional log conversion circuit. This gain cell circuit, used as an element of an active filter circuit, is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 61-224715. A log conversion circuit on an input stage includes transistors Q1, Q2, Q3 and Q4, degeneration resistors RE1 and RE2 and a current source I1. An antilog conversion circuit includes transistors Q5 and Q6 and current sources I2, I3 and I4.
In the log conversion circuit, input signals Vin (Vin.sup.+, -Vin.sup.-) applied to the bases of the transistors Q1 and Q2 are voltage-current converted by the transistors Q1 and Q2. Emitter currents of the transistors Q1 and Q2 are respectively supplied to the collectors of the transistors Q3 and Q4 through the resistors RE1 and RE2.
A collector current Ic of the diode-connected transistors Q3 and Q4 and a base-emitter voltage Vbe have the following relationship: EQU Vbe=.alpha..multidot.1n(Ic).
Collector currents of the transistors Q3 and Q4 are therefore log-converted and output as base-emitter voltages of the transistors Q3 and Q4. The output voltages are received by the bases of the transistors Q5 and Q6 of the antilog conversion circuit on an output stage of the gain cell circuit. As a result, the signals Vin input to the log conversion circuit of the input stage are linearly converted and output from the collectors of the transistors Q5 and Q6.
FIG. 2 shows an equivalent circuit of the log conversion circuit on the input stage shown in FIG. 1. Since the log conversion circuit as shown in FIG. 1 performs a differential operation, the equivalent circuit shown in FIG. 2 is indicated in a half-circuit form. When the resistance of the degeneration resistors RE1 and RE2 is represented by "re" and the current of the current source I1 is represented by "i (mA)", a maximum range of an input voltage, i.e., a maximum amplitude range Vinp-p of the input signal Vin, is represented by the following equation: EQU Vinp-p=100.multidot.re(i(mA)/26)(mV) (1)
As is evident from the equation (1), in order to increase the range of the linear operation of the gain cell circuit, it is necessary that the current "i" or the resistance "re" be increased. However, an increase in the current "i" is not desirable, since it increases the amount of power which is consumed when no signal is generated. On the other hand, if the resistance "re" is increased, the source voltage must be greater in accordance with the resistance, since the voltage drop in the resistors RE1 and RE2 also increases.
In the log conversion circuit shown in FIG. 1, the transistors Q1 and Q3 (Q2 and Q4) and the resistor RE1 (RE2) are connected in series between a power source Vcc and a ground terminal GND. Hence, the source voltage must be higher than the sum of the base-emitter voltage of the transistors Q1 and Q2 (Q3 and Q4) and the voltage drop in the resistor RE1 (RE2) due to a DC bias current of the transistors Q1 and Q2 (Q3 and Q4). Since the voltage drop in the resistors RE1 and RE2 is great as described above, the log conversion circuit is not suitable for a low-voltage operation. In other words, since the voltage amplitude range of an input signal, which can be log-converted, is restricted due to the voltage required to operate the transistor, an input signal of an electronic device driven by a low source voltage (e.g., a portable communication device) cannot have a sufficiently wide voltage amplitude range.
As described above, the conventional log conversion circuit is not suitable for a low-voltage operation, since the source voltage must be higher than the sum of the base-emitter voltage of two transistors and the voltage drop in a resistor due to a DC bias current of the transistors. Accordingly, when the source voltage is not sufficiently high, it is impossible to perform a log converting operation over a wide range of voltage amplitude of an input signal.